This invention relates to a bar code reading apparatus using a visible rays.
Bar code reading devices are being utilized in many fields, such as, at the checkout counters in supermarkets, the management of inventories, or in the management of a manufacturing process in a factories, etc.
The demand for a bar code reading apparatus using a laser diode radiating a red beam has been increasing, because of the requirements for credibility, maneuverability, and miniaturization.
FIG. 1 shows a block diagram of a currently used bar code reading apparatus. A light beam source 1, such as a light emitting diode (LED) or a laser diode, radiates a light beam onto a bar code symbol 3 via a scanner 2, such as a mirror. A reflected light beam 4 is supplied to a photo detector 5, such as a photo diode, where the reflected light beam 4 is converted to corresponding electric signal 6. The electric signal 6 is amplified by an amplifier 7, and the amplified electric signal 8 is supplied to a wave forming circuit 9. The wave forming circuit 9 supplies a pulse train signal 10 corresponding to the amplified electric signal 8. A decoder circuit 11 decodes the pulse train signal 10, and supplies an output signal 12 to following stages (not shown).
According to the bar code reading apparatus depicted in FIG. 1, the bar code symbol 3 is scanned by the scanner 2 or by a manual scanning operation of the light beam. The reflected light beam 4 is eventually converted to a decoded signal by the decoder circuit 11.
FIGS. 2(a) and 2(b) show graphs representing a relationship between a wave length of the light beam and a reflection index of a colored bar code symbol. As will be apparent from the graph identified as "Red" in FIG. 2(a), a light beam, which is approximately 600 nm or more in wave length, is reflected when the color of bars in the bar code symbol is red. On the other hand, if the light beam is less than 600 nm in wave length, the light beam is almost absorbed by red bars. As a result, the bar code reading apparatus can identify a contrast between red bars and spaces on a bar code symbol, if the wave length of the light beam is less than 600 nm. In particular, such a light beam is reflected enough by spaces, and is absorbed enough by red bars.
Similarly, orange bars reflect the light beam, when the wave length of the light beam is approximately 550 nm or more. Yellow bars reflects the light beam, when the wave length of the light beam is approximately 500 nm or more.
As will also be apparent from the graphs shown in FIG. 2(b), it is understood that it is necessary to provide a light beam having the wave length between 620 nm through 700 nm to read the bar code symbol containing green, blue, or magenta bars, because these three colored bars absorb the light beam having a wave length between 620 nm and 700 nm.
As aforementioned, it is understood that there is a close relationship between the color of the bars of a bar code symbol and the wavelength of the light beam being projected onto the bar code symbol. It should be noted that the wave length of a red laser beam is not capable for reading red, orange or yellow colored bars of a bar code symbol.
A current bar code reading apparatus uses a red laser beam, and the power of the beam is limited up to 1 mW pursuant to a safety standard, such as International Electrotechnical Commission (IEC) standard, in order to protect human eyes. On the other hand, in order for human eyes to recognize whether light beams are being projected onto a bar code symbol, a laser diode is required to radiate light beams having a relatively short wave length, such as 635 nm through 670 nm, which are visible by human eyes. However, it is the fact that the intensity of a red laser beam is still insufficient for human eye recognition under strong white light, such as fluorescent light.
Moreover, the value of the bar code reading apparatus, as well as the number of items being handled by a bar code system, has been increasing. Therefore, the quantity of the information represented by bar code symbols had to be increased. In order to increase the capacity of information, two dimensional bar code systems have been developed. Two dimensional bar code symbols contain horizontal information and vertical information. A bar code reading apparatus for reading the two dimensional bar code symbols must scan a light beam diagonally across the bar code label, such as a TV scanning method. It should be apparent that this is a shortcoming of a limitation of scanning direction. As a result, the error rate of scanning two dimensional bar code symbols is rather high.
Another example of bar code reading apparatus is described in the U.S. Pat. No. 5,361,158, issued to Tang on Nov. 1, 1994. This patent discloses a multiple source optical scanner, which employs a plurality of scanning light sources. According to the description in the patent, the laser lights are of different wave lengths. Specifically, the wave lengths described in the patent are 635 nm, 670 nm, and 780 nm. Such beams belong to red or infrared beam region. Thus, the technology described in the patent still has a shortcoming of not enabling human eyes to recognize whether light beams are being projected onto a bar code symbol.